The Forbidden City is a royal palace in the Ming and Qing Dynasties. "Red walls and yellow tiles" are the classic impressions of the Forbidden City. Among them, the red wall refers to the palace wall in the Forbidden City, and its appearance is mostly decorated with red. Red has many beautiful meanings such as sunshine, warmth, enthusiasm, and festiveness. It is the color preferred by the Chinese nation, so it is also called Chinese red. The red wall of the Forbidden City presents a magnificent and magnificent aesthetic conception, with a unique visual beauty. Not only that, but the use of colors, construction techniques, and even legends about the red wall of the Forbidden City all contain relatively rich scientific content.

　Scientific research shows that the main component of the red clay of the Forbidden City's red wall is iron oxide (Fe2O3). This material has a strong coloring ability, is relatively stable in the atmosphere and sunlight, and has good corrosion resistance and high temperature resistance. In order to make Chinese red more firmly attached to the surface of the palace wall, clever ancient craftsmen mixed glutinous rice and alum into the red wall. The main component of glutinous rice is starch, which can form a highly attractive spatial grid, and has a good control effect on the size and shape of the lime bottom layer, which is conducive to the compactness of the crystals, thus facilitating the bonding of laterite to the bottom ash. Alum, also known as alum, alum stone, etc., is processed and refined from the aluminum potassium sulfate mineral alum stone. After alum is mixed with lime, ettringite is formed, and its solid-phase volume expansion plays a certain role in compensating the drying shrinkage of the mortar, so it is beneficial to improve the compressive strength, water resistance and freeze-thaw resistance of the mortar. The ingenious use of materials such as rice, alum, and top-grade red clay makes the red wall of the Forbidden City present the beautiful color of Chinese red, which can last for several years without falling.

　　There are many legends about the red wall of the Forbidden City, the most famous of which is the "Phantom of the Palace Lady" in the 1990s. Some tourists who visited the Forbidden City said that they saw the image of the "ancient palace lady" on the red wall when the lightning struck. From the perspective of material science, Fe2O3 exists in two forms: α-Fe2O3 and γ-Fe2O3. The main component of the red wall overlay ash of the Forbidden City belongs to α-Fe2O3, which is stable in nature. However, in thunderstorms, lightning will produce a small amount of hydrogen, and α-Fe2O3 and hydrogen can undergo a reduction reaction to form iron tetroxide (Fe3O4). Fe3O4 reacts with oxygen at high temperature to produce γ-Fe2O3. γ-Fe2O3 has the function of magnetic recording and is one of the important materials of magnetic tape. In ancient times, when a palace lady walked inside the red wall, if she happened to encounter thunder and lightning, the interaction between the lightning and the magnetic field could produce γ-Fe2O3 on the surface of the red wall, so that the "Phantom of the Palace Lady" was "recorded" on the red wall. The process is similar to how magnetic tape preserves the magnetic signals of video and audio under the action of an electromagnetic field. Hundreds of years later, on the premise that the ash on the red wall is still well preserved, when tourists visit in a thunderstorm, it is very likely that the lightning and the magnetic field will interact again, making the "Phantom of the Palace Lady" "recorded" on the red wall reappear. This process is similar to the magnetic signal of a magnetic tape under the action of an electromagnetic field to restore video and audio. From this, it can be considered that the discovery of the red wall by "Phantom of the Palace Lady" is not without scientific basis, and it is possible.